One type of conventional stringer has a T-shaped cross-section with a foot and a web. Such T-shaped stringers are usually made up of two L-shaped preforms having the same thickness in the foot and in the web, although there are also L-shaped preforms in which the web area is thicker than the foot area. This difference in thickness is because some stringers have to have higher inertias, so they have an additional number of fabrics as reinforcement in the web area. In both cases, they are manufactured by means of a process comprising a first tape laying step, a second shaping step and a third curing step.
Laying tape comprises stacking bands of pre-impregnated material, for which purpose an ATL machine is commonly used. In this step, the machine deposits bands of pre-impregnated material (carbon fibers pre-impregnated with a resin) on top of others until obtaining the desired laminate with the desired fiber orientation.
The second step for conventionally manufacturing stringers comprises shaping laminates to obtain L-shaped preforms, which will subsequently be attached to one another in twos to obtain a T-shaped stringer. Pressure and a temperature below 100° C. are normally applied when shaping. The purpose is to reduce the viscosity of the resin in order to give the desired shape to the laminate. The resin is never cured.
The third step of the conventional method for manufacturing stringers comprises attaching the preforms to one another such that they form the T-shaped stringer to subsequently cure it. The curing process can be carried out in different ways. The stringers can be placed on the overlay and cured at the same time as the overlay (co-curing), cured separately from the overlay and subsequently bonded on the overlay in an already cured state (secondary bonding), placed while fresh on the overlay in an already cured state and cured in the bonding cycle (co-bonding), or placed in an already cured state on the overlay before drying and the overlay being cured at the same time the stringers are bonded (also co-bonding). Pressure and temperature higher than those applied during shaping are applied when curing because the purpose is to cure the resin and to get the resin to be redistributed in order to fill the cavities that may exist in the part, thereby reducing porosity.
A problem with this conventional method of manufacture is that the stringers manufactured according to this method have beaked excess material at the end of the web that does not withstand loads and is therefore useless weight. Today, this excess material is machined and material is pulled off until obtaining an upper web surface of the stringer that is horizontal and planar. This machining operation can damage the end of the web of the stringer and, even while not damaging it, the resulting structure does not perform well in response to impacts, which can cause peeling in this area.
Another problem with the conventional method arises from the need to identify the damage that may occur given that this area is susceptible to receiving impacts. The dark gray color of these parts made of composite material does not allow the detection, so as of today, the upper portion of the web of the stringer is painted using paint with a color that is lighter than the color of the composite material in order to identify the damage. The problem with this solution is that it is a time-consuming process because since only the upper area of the web of the stringer has to be painted, the rest of it must be previously covered.